1. Field of the Invention
The present invention relates to a method of disposing of radioactive metal waste using melting decontamination, and more particularly, to a method of disposing of metal waste, in which metal waste generated in nuclear fuel processing or production facilities is sorted by predetermined sorting criteria, followed by performing melting decontamination, subjecting the resulting ingots to gamma spectroscopy to measure radioactivity of only U-235 among uranium isotopes, calculating total radioactivity of the ingots, and efficiently and rapidly determining whether clearance of the ingots is possible.
2. Description of the Related Art
Industrial waste composed mainly of iron metals such as stainless steel and carbon steel is treated as a very important resource, and the rate of recycling thereof is considerably higher than other waste.
Typically, recycling of waste aims to resolve the dearth of natural resources and environmental problems such as air, water and soil contamination due to waste. However, in the case of metal waste, because the cost required to recycle resources or waste which has been re-treated is much smaller than the production cost of products using new natural resources, disposal of metal waste without recycling is regarded as a big loss in terms of environmental protection and economic benefits.
Also metal waste generated in nuclear facilities may be reproduced using a recycling process like other industrial metal waste. However, there is a potential for such metal waste to be radioactive because of artificial neutron irradiation or for the surface or volume thereof to have been contaminated by the radioactive materials used in nuclear facilities. Hence, when this waste is discharged to the market without appropriate restrictions, the public may be indiscriminately exposed depending on the recycling of contaminated metals. Accordingly, all metal waste generated in the radiation controlled zones of nuclear facilities is considered to be subject to restrictions in principle. However, if the same restriction rule is applied to the case where radiological effects on the public and the environment are considerably small because the concentration of radionuclides in metal waste is very small, the economic and social costs may be unnecessarily increased. In accordance with domestic nuclear-relevant act, only in the case where the concentration of radionuclides of metal waste is below a predetermined level (a disposal limit), namely where radiological effects on the public and the environment due to the recycling of metal waste are not more than a clearance limit defined by the nuclear-relevant act, is the waste deregulated so as to be disposed of (recycled). The related radiation safety management and radiological risk assessment are an absolute requirement of regulatory agencies, thus minimizing radiological effects on the public and the environment resulting from clearance.
Because metal waste such as filter frames, powder drums for natural uranium, nuts, bolts and metal scrap generated in nuclear fuel processing and production facilities are expected to be contaminated with uranium compounds such as UO2, UO2F2 or U3O8, these are regarded as radioactive waste and have to be regulated. However, the case where the concentration of the radioactive contaminant in the waste is not more than a clearance limit as mentioned above is deregulated, making it possible to carry out clearance via recycling.
Meanwhile, metal waste which is of the flat panel type and has a geometrical shape that is comparatively simple and which has a smooth surface or which has shapes similar thereto may be recycled using only surface decontamination. The real-time determination of radioactivity after a decontamination process is possible using both direct measurement using a surface contamination meter that operates in nuclear fuel processing plants and indirect measurement using smear. Thus, the surface contamination of metal waste having complicated geometrical shapes such as nuts or bolts cannot be subjected to direct measurement and smear measurement is also not easy, making it very difficult to decontaminate such metal waste and measure the radioactivity thereof.
In the case where metal waste having a complicated structure is heated to high temperature and melted, because the radioactive material in metal is uniformly distributed in the medium and the nuclear fuel material which is a contaminant of the melt moves into the slag, metal waste on which it is difficult to perform surface decontamination and direct measurement may be subjected to melting decontamination so that the volume thereof is reduced and uranium materials are removed from the metal medium, making it possible to carry out clearance.
Thorough research into melting decontamination techniques of metal waste containing radioactive materials is ongoing both within and outside the country. Particularly in the case where contaminants are nuclear fuel (uranium nuclides), most radioactive contaminants are reported to move into the slag upon melting. Although the decontamination effects thereof may ingot depending on initial contamination conditions and operating conditions including the melting additive used, the type of melting furnace, etc., the amount of uranium moving into the slag upon melting decontamination of the metal is at least 1000 times the amount of uranium that moves to ingots. This trend is reported to increase as the initial contamination becomes higher.
For example, Korean Patent No. 10-1016223 discloses a melting decontamination system of radioactive metal scrap, in which radioactive metal waste generated in nuclear facilities, containing U-238, Ce-144, Cs-134, Cs-137, Sr-89, Sr-90, Ni-63, Co-58, Co-60, Cr-51, etc. as nuclides to be decontaminated, are subjected to melting decontamination, so that decontaminated ingots are separated from radioactive slag, and the decontaminated ingots are recycled and the radioactive slag is treated as radioactive waste.
However to perform clearance of the ingots produced using melting decontamination, radioactivity of the ingots should be proven to be less than the disposal limit defined by nuclear-relevant act. To this end, measurement of the radioactivity of the ingots must be reliable.
Prior to melting decontamination, metals have surface contamination characteristics, whereas ingots produced using melting decontamination have volume contamination characteristics. Uranium nuclides which are radioactive contaminants are alpha emitters, so that alpha rays emitted upon the decay of uranium react with the medium and are mostly self-absorbed. Thus conventional radioactivity analysis methods using alpha nuclide surface contamination measurement may not be effective at analyzing the radioactivity of ingots.
Therefore the present invention is intended to provide a method of efficiently disposing of radioactive metal waste, in which the radioactivity of ingots produced using melting decontamination may be confirmed to be sufficiently uniform, and determination of whether clearance of ingots having volume contamination characteristics is possible based on such radioactivity results may be rapidly made.